Structures such as superconducting magnets are typically cooled by at least partial immersion in a bath of liquid cryogen. For superconducting structures, such as superconducting magnet coils for MRI (Magnetic Resonance Imaging) or NMR (Nuclear Magnetic Resonance) scanners and the like, the cryogen used is liquid helium. Typical cryogen baths hold volumes of liquid helium in the order of 1000 liters.
During the final stages of manufacture, a cryogenically cooled superconductive magnet is subjected to training cycles. That is, current is repeatedly ramped up until the magnet holds the current without quenching. Since one or more quench events are likely to occur during these training cycles, a significant amount of liquid cryogen is consumed.
The expression ‘ramping up’ refers to the progressive introduction of current into a superconducting magnet. Once ramped to full current, producing the full magnetic field, a superconducting magnet will remain in this state until ‘ramped down’, that is, the current is removed from the magnet and the generated magnetic field falls to zero.
The increasing cost and global shortages of liquid helium necessitate reductions of the quantity of liquid helium used in cooling superconductive magnets to low temperature and lost in training cycles, as well as amount of helium stored in the cryogen baths. There are a number of patents proposing spacers to reduce the required helium volume, or various types of heat links for cooling superconductive parts of the magnet with reduced helium level (e.g. EP1522867), avoiding the need for immersion in a cryogen bath. In some examples, a relatively small quantity of helium is circulated around a cooling loop: a thermally conductive pipe partially filled with liquid helium and in thermal contact with the cooled equipment, in conjunction with a cryogenic refrigerator arranged to keep the helium in its liquid state (WO9508743).
All these solutions require additional expensive components. They increase risk of failure, e.g. leaking cooling pipes. They can be potentially dangerous in case of a quench. For example, the spacers restrict gas flow paths, or coils overheat as cooling loops can not transfer the quench energy fast enough. As the maximum amount of helium that could be stored in the magnet is reduced, special solutions for keeping the magnet cold during transportation are required, such as tanks with frozen nitrogen as described in copending United Kingdom patent application GB 0515936.3.